file-helpers: Add ephy_file_open_uri_in_default_browser()

Helper function to open the given uri in the default browser
embed: Add ephy_embed_utils_urls_have_same_origin()

Helper function to check if two URLs have the same origin
window: Simplify the code to open external uris in default browser when in app mode

Use ephy_embed_utils_urls_have_same_origin() to check if the target URL
is external or not and ephy_file_open_uri_in_default_browser() to open
the URL in the default browser if it's external.
app-mode: Make sure external links are openend in the default web browser

Some sites like gmail, open external links by first opening an empty
window and then loading the URL in that window. In that case we need to
wait until we know the URL to decide whether to show the window, or open
the request in the default browser.

https://bugzilla.gnome.org/show_bug.cgi?id=658395
window: Ignore new-window-in-tabs setting in application mode

There are no tabs in application mode.

https://bugzilla.gnome.org/show_bug.cgi?id=712402